1. Field of the Invention
This invention relates to a method of producing grain oriented silicon steel sheets suitable as a core material for use in transformer and other electric machinery and having less iron loss.
2. Related Art Statement
As a means of reducing iron loss of the grain oriented silicon steel sheet, there have hitherto been adopted 1) a method of increasing Si content, 2) a method of thinning product thickness, 3) a method of refining secondary recrystallized grains, 4) a method of reducing impurity content, 5) a method of highly aligning secondary recrystallized grains into (110)[001] orientation (called as Goss orientation), and the like. Among them, the method of increasing the Si content is not adaptable as an industrial production method because when the Si content exceeds 4 wt % (hereinafter shown by % merely), the cold rolling property is considerably degraded.
On the other hand, there are proposed various methods for reducing the product thickness. For example, in the grain oriented silicon steel sheet containing AlN as an inhibitor, Japanese Patent laid open No. 58-217630 and No. 59-126722 disclose a method wherein products having a thickness of 0.15-0.25 mm are obtained by adding Sn, Cu to the steel composition. Furthermore, in the grain oriented silicon steel sheet containing MnSe, MnS as an inhibitor, Japanese Patent laid open No. 62-167820, No. 62-167821 and No. 62-167822 disclose a method wherein products having a thickness of 0.15-0.25 mm are obtained and an average grain size after secondary recrystallization is within a range of 1-6 mm.
In the method of adding Sn and Cu to the grain oriented silicon steel sheet containing AlN as a main inhibitor as disclosed in Japanese Patent laid open No. 58-217630 and No. 59-126722, however, a relatively high magnetic flux density was obtained, but it could not be said that the improvement of iron loss was sufficient. For example, in Table 5 of Japanese Patent laid open No. 59-126722, the iron loss is 0.85-0.90 W/kg as W.sub.17/50, which can not be said to be a satisfactory value. Furthermore, a proper reduction in final cold rolling exceeds 80% in the grain oriented silicon steel sheet containing AlN as a main inhibitor, but when the product thickness is not more than 0.23 mm, the secondary recrystallization becomes unstable and the probability for obtaining products having less iron loss rapidly decreases.
In the method of Japanese Patent laid open No. 62-167820, No. 62-167821 and No. 62-167822 aiming at the thinning and refining of crystal grain in the grain oriented silicon steel sheet containing MnSe, MnS as an inhibitor, the magnetic flux density is low as compared with the grain oriented silicon steel sheet containing AlN as a main inhibitor, and hence the hysteresis loss is poor. However, such a method is advantageous in the fine division of crystal grain, and hence the total iron loss is 0.83-0.88 W/kg as W.sub.17/50 as shown in Table 2 of Japanese Patent laid open No. 62-167820, which is substantially equal to that in the case of using AlN as a main inhibitor.
However, it is hardly said that such a level of iron loss is fully satisfactory. Moreover, the optimum reduction in cold rolling is 55-88% in the grain oriented silicon steel sheet containing MnSe, MnS as a main inhibitor, so that even when the product thickness is not more than 0.23 mm, the secondary recrystallization becomes considerably stable as compared with the grain oriented silicon steel sheet containing AlN as a main inhibitor.